People who grow marijuana illegally in the backwoods of Northern California use large amounts of rat bait to protect their plants — and these chemicals are killing several species of wild animals, including rare ones, biologists say.

Here’s what happens: The growers plant their marijuana in remote locations, hoping to elude detection. They irrigate their plants — with water from streams — which lures animals looking for water. Rodents chew the flourishing plants to get moisture, which kills the plants. Researchers believe that’s the prime reason growers use the poisons.

“The problem is we have wild rodents out here that are going to eat the rat poisons, and then they become little time bombs,” says Mark Higley, wildlife biologist in the area for the Hoopa Valley Tribe. “They don’t die for seven to 10 days, maybe two weeks. And they stagger around and then become easy prey for Northern spotted owls, fishers, foxes, bobcats.”

Predators that eat the poisoned rodents often become weakened or die.

In studies of fishers (a rare relative of the weasel), nearly all the tracked animals that died in recent years — from the southern Sierra to the northern tip of the state — had rat poison in samples of their tissue.

And on the Hoopa reservation alone, postmortem exams have shown that rat bait caused nearly a third of the deaths of male fishers in recent years.

“To know that such a high percentage of the fisher population across the entire range in California was exposed to these rat poisons — and knowing that it’s not just fishers, but all the other carnivores that have got to be exposed — it just seems absolutely criminal,” says Higley.

One winter day, Higley, some of his fellow researchers and I scramble through a snow-covered wilderness. On a remote hillside, Higley finds a withered plant.

“Here’s a marijuana plant that kept growing after it was cut,” he says.

It’s part of the remains from a raid last summer. A law enforcement team swooped in by helicopter and found 8,000 marijuana plants here, all between 3 feet and 6 feet tall, as well as lots of leftover rat poison and empty poison containers — trash left by the growers. (Most of the plants were hauled away, but some of the poison got left behind.)

Higley says this has become a common scene in the tribe’s forests and throughout Northern California.

Sometimes the growers’ poisons seem intended not for rodents, but for bigger animals. In another bust last summer, for example, in a national forest near the reservation, sheriff’s deputies found a dead fisher. Above it, hot dogs laced with a toxic insecticide hung on large fish hooks.

The fisher had chunks of hot dog in its esophagus and stomach; it died of acute poisoning, says Higley, who accompanied law enforcement on that bust.

He and other biologists have a good sense of how these poisons are affecting fishers, because they’ve been studying the cat-size animals closely — the federal government has been considering putting fishers on the endangered species list. So biologists have been documenting their numbers and keeping track of how each animal dies. Researchers track each fisher with a radio collar; when an animal stops moving, they race into the forest to find its body before a predator does.

In fact, it was a dead fisher that first clued scientists into the widespread use of poisons by illegal pot growers. Blood had pooled in the animal’s abdomen, a telltale sign of poisoning with anticoagulant rat bait.

At first the researchers were puzzled; fishers tend to live in remote areas, far from human homes. But once they learned of the poison used by illegal pot farmers, the scientists looked back through their collection of tissue samples from fishers that had died in previous years. Nearly all those samples, they discovered, contained traces of various rat poisons.

“It was just devastating,” Higley recalls.

Since then, Higley has documented 19 illegal pot farms in the forests on the Hoopa reservation. He says the use of these dangerous chemicals on the reservation is particularly galling because the Hoopa Valley tribe banned all toxic chemicals decades ago to protect natural resources.

Aaron Pole, a wildlife technician and a tribal member, says he’s come across illegal marijuana farms when tracking fishers and Northern spotted owls in the backwoods of the reservation. “I’m enraged,” Pole says. “Why don’t [these people] go somewhere else and grow?”

Pole has also helped clean sites after busts. He’s seen how growers encircle their marijuana plants with rat poison and use large amounts of insecticide and fertilizer.

On the day of our visit, a big pile of trash remains from a cleanup a couple months earlier. It includes empty fertilizer bags and lots of irrigation hose the growers used to bring water from a stream. The area is so remote that a helicopter will be needed to come pick up the remaining mess — but funds for cleanups are hard to come by.

Only seven of the 19 known marijuana farms on the reservation have been cleaned up so far, according to Higley.

“It’s pretty selfish,” says Pole. “They come in. They reap the benefit of, you know, some quick cash. And all the environmental damage is happening here and we have to deal with it.”

The fishers are a special concern to the tribe, whose members use fisher skins as arrow quivers in a traditional healing dance. But the tribe also worries about poisons getting into fish and game — and into the rivers where children play.

Wildlife ecologist Mourad Gabriel, of the nonprofit Integral Ecology Research Center, has been tracking the problem all over the northern half of California. In his lab in a logging town near the coast, he opens a locked freezer and pulls out the corpse of a furry animal.

“So you can see here, it’s still a little frosty,” he says, “but this is a beautiful, beautiful male fisher.”

The animal was found dead near Yosemite National Park, several hundred miles from the Hoopa reservation. Gabriel tells me he’ll take it to the University of California, Davis, for an examination to determine the cause of death.

But these days, whenever a dead fisher turns up, Gabriel suspects rat poison. Like Higley, Gabriel accompanies law enforcement officers during busts of illegal pot farms — sometimes he’s lowered from a helicopter to the remote setting.

Afterward, the scientists hike back into these spots and haul out whatever rat poisons they can find.

Gabriel sets up motion-sensing cameras to see which animals visit these sites and are, potentially, exposed to the poisons. The wildlife his photos have caught includes fishers, mountain lions, gray fox, deer and bears rummaging through trash piles — bottles of insecticide, fertilizer and other dangers.

Soon after my visit, Gabriel got back results from the autopsy of that frozen fisher. It did die from consuming rat poison (or a poisoned rat or other small animal).

And earlier this month, Gabriel suffered a very personal blow — his pet dog was poisoned with the same kind of rat bait and died. Gabriel thinks the poisoning was malicious. The local sheriff is investigating.